The present invention relates to a low-noise printing head for use in wire-dot printers, and more particularly, to an improvement on a printing head having print pins which are driven using electromagnetic force.
Of wire-dot printers presently used as output apparatus for computers, a clapper type printer is mainly used. In a clapper type printer, an armature is disposed at the end of a wire. The wire end is hammered with the armature against the force of a spring while it is attracted by electromagnetic force. The tip of the wire is then projected out of the print head to record a dot on a recording paper through an ink ribbon.
Since noises are produced when armatures strike the wire ends, development of a low-noise wire-dot printer has been desired for long. An electrodynamic type printing head as disclosed in Japanese Patent Unexamined Publication No. 60-206, 669, corresponding to U.S. Pat. No. 4,600,322 to Vermot-Gaud et al, has a bright future for use in a low-noise wire-dot printer. In an electrodynamic type printing head, conductive actuator pins fixed at their ends are disposed within a magnetic field. When current flows through the actuator pins, the generated electromagnetic force deforms the actuator pins to thereby project outwardly of the print head print pins fixedly mounted at the tip of the actuator pins. Since the printing head of this type does not use impact force to move a print pin, noises are rarely produced by the printing head. The structure thereof is simple.
However, since the above-described conventional actuator pin has a curved shape, sufficient rigidity (spring constant) cannot be obtained so that a high speed print is not possible.
Meanwhile, in order to improve a print quality, it is necessary to use a number of pins. For example, a clapper type wire-dot printer used mainly in this field has a number of print pins, with 9 to 12 pins disposed in a stagger fashion in each of two arrays along the height (about 3 mm) with respective to a print character. However, the above-described low-noise electrodynamic type printing head has actuator pins which project laterally relative to each other. Therefore, if actuator pins are disposed in a stagger fashion in two arrays at a same interval as that of the clapper type, adjacent pins may contact each other. To avoid this, an actuator pin having a diameter smaller than 0.1 mm may be used. However, this makes a dot size too small for obtaining a good quality of print. Besides, there arises a problem of a low mechanical strength of the actuator pin.
In order to avoid contact of adjacent two pins, pins may be disposed as shown in FIG. 9 which shows a horizontal cross section of a printing head. A printing pin 4 is fixedly mounted at the tips of two actuator pins 2 and 3 constituting a first array. Similarly, a printing pin 7 is fixedly mounted at the tips of two actuator pins 5 and 6 constituting a second array. The ends 2a of actuator pins are fixed at a head frame 8 in which the above elements are housed. Print pins 4 and 7 in the respective arrays are movably inserted into holes formed in the head frame 8.
As current flows through a pair of actuator pins 2 and 3, electromagnetic force is generated in the direction indicated by arrows or in the opposite direction, depending on the direction of the current flowing through the actuator pins. As a result, the actuator pins 2 and 3 are deformed moving apart from each other or coming near each other to thus effect a straight movement of the printing pin 4. For the stagger arrangement of print pins 4 in two arrays, it is necessary to ensure a sufficient lateral space among them so as to avoid any contact of actuator pins while they are elastically deformed. With this arrangement, however, a distance between print pins 4 and 7 becomes large so that a high density arrangement of print pins is not possible. Also in this case, adjustment of ink dot positions during printing becomes difficult, and the lateral length of the printing head becomes long.
If a printing head contacts an ink ribbon, a recording paper will be blurred. It becomes necessary, therefore, to maintain a clearance larger than 300 microns between the printing pin and the recording paper. With a larger clearance between the printing pin and the recording paper, not only a recording paper can be easily set at the printer, but also it becomes possible to use a plurality of sheets of pressure sensitive paper for multiple print.
The amount of movement of a printing pin of the above-described electrodynamic type printing head is proportional to the magnetic flux density, current value and length of actuator pin. A large permanent magnet is required for a strong magnetic flux, and a large power consumption for a large current value, thus leading to disadvantages. Consequently, it is better to use long actuator pins. However, with a conventional curved actuator pin which extends laterally, it is necessary to employ a large lateral dimension in order to obtain a sufficient length of actuator pins. However, as the lateral dimension becomes large, the printing head becomes bulky and a large permanent magnet is required for covering such a broad magnetic field.